What’s Behind the Door in the Pyramid?

The Great Pyramid is thousands of years old and stands today as the last of the seven wonders of the ancient world, but everything about it is shrouded in mystery. How was it built? Did the ancient Egyptians discover the secrets of complex mathematics, astronomy, geology, and engineering? Why has no mummy ever been found in any pyramid, the supposed burial tombs of the Pharaohs? And what's behind the secret door in the Great Pyramid, which hasn't yet been opened a decade after its discovery?